1. Field of the Invention
The invention relates to a sewage dewatering process and equipment to carry out the process.
2. Background Art
Sewage is composed of the liquid and water-carried wastes from residences, commercial buildings, industrial plants, and institutions, together with any groundwater, surface water and storm water which may be present. The terms "wastewater" and "sewage" are sometimes used interchangeably herein.
The composition of sewage depends on its origin and the volume of water in which the wastes are carried. Sewage which originates entirely from residential communities is made up of excreta, bathing and washing water, and kitchen wastes. Other wastes can be present from rural/agricultural sources and/or industrial or commercial establishments.
Modern sewage treatment is generally divided into three phases: primary, secondary and tertiary. Each of these steps produces sludge, which can be disposed of or used for various purposes.
Primary treatment, or plain sedimentation, removes only the settleable solids from sewage. A modern system for primary treatment entails collecting the sewage, conveying it to a central point for treatment, using both screens to remove large objects and grit chambers to remove grit, and using primary sedimentation tanks to remove the suspended settleable solids. This type of system produces about one third of a gallon of wet sludge per person per day, and facilities for handling and disposing of the sludge are also needed. Primary treatment reduces the concentration of suspended solids by about 60 percent and reduces the BOD (biochemical oxygen demand) by about 35 percent.
Secondary treatment involves the addition of a biological treatment phase following plain sedimentation. At best, this treatment removes about 85 to 95 percent of the organic matter in sewage. It has little effect on dissolved materials or on the nutrients that stimulate the growth of algae in the receiving waters. It also discharges all of the nutrients and dissolved solids, as well as any contaminants which may be added to the water by industrial plants.
There are two basic methods of often used in modern secondary treatment, that is, the trickling filter and the activated-sludge processes. In small communities, secondary treatment is sometimes accomplished by either the trickling-filter method or the contact bed method, but usually used is the sand filter method. In larger communities, secondary treatment is generally accomplished by the activated-sludge process.
Sand filters are beds of fine sand, usually 3 feet (1 meter) deep, through which the sewage slowly seeps. As it seeps through the sand, the organic matter is decomposed and stabilized by the microorganisms in the sewage. Sand filters require about 4 acres (1.6 hectares) of sand beds for each thousand people. Because of this large space requirement, sand beds have obvious disadvantages. Also, the time required for the sludge to be formed and dried usually takes weeks. This long drying time means that large surface areas of sand beds have to be used to achieve drying with the attendant large cost of constructing, operating and maintaining the sand beds. Rain adds time to the drying function of sand beds, since the sand beds usually are without any roof or other top covering. Covered sand beds require less area than do uncovered beds but still take weeks to achieve drying and have a higher construction cost. Nowadays, about 90 percent of smaller municipalities use sand beds to dewater sewage coming from primary treatment units. The main purpose of sand beds is the reduction of the water content in the primary-treated sewage.
A contact bed, composed of many layers of stone, slate or other inert material, provides a relatively large surface area for the growth of microorganisms. It operates on a fill-and-draw basis, and the organic matter delivered during the fill period is decomposed by the microorganisms on the bed. The oxygen required by the microorganisms is provided during the resting period, when the bed is exposed to the air.
In the trickling filter system, the sewage is applied to the filter through rotary distributors and, then, is allowed to trickle down over large stone or plastic beds that are covered with microorganisms. The beds are not submerged and, thus, air can reach the organisms at all times. The area requirements for trickling filters are about 5 to 50 acres (2 to 20 hectares) per million people.
In the activated-sludge process, heavy concentrations of aerobic microorganisms, called biological floc or activated sludge, are suspended in the liquid either by agitation which is provided by air which is bubbled into the tank or by mechanical aerators. Final sedimentation tanks are needed to separate the floc material from the flowing liquid. Most of the biologically active sludge, then, is then returned to the aeration tank with which to treat the incoming water. The high concentration of active microorganisms which can be maintained in the aeration tank permits the size of the treatment plant to be relatively small, about 1 to 5 acres (0.1 to 2 hectares) per million population.
Tertiary treatment is designed for use in areas either where the degree of treatment must be more than 85 to 95 percent or where the sewage, after treatment, is reused. It is mainly intended to further clean or polish secondary treatment plant effluents by removing additional suspended material and by lowering the BOD, generally by filtration. This polishing, however, has little impact on the dissolved solids, including the nutrients, synthetic organic chemicals, and heavy metals. To eliminate these constituents of sewage, other methods of treatment have been devised. These processes include coagulation and sedimentation, precipitation, adsorption on activated carbon or other adsorbents, foam separation, electrodialysis, reverse osmosis, ion exchange and distillation.
Sludge is the semiliquid mass removed from the liquid flow of sewage. Sludge will vary in amount and characteristics with the characteristics of sewage and plant operation. Sludge from primary treatment is composed of solids usually having a 95 percent moisture content. The accumulated solid materials, or sludge, from sewage treatment processes amount to 50 to 70 pounds (22 to 31 kg) per person per year in the dry state or about one ton (0.9 metric ton) per year in the wet state. Sludge is highly capable of becoming putrid, and can, itself, be a major pollutant if it is not biologically stabilized and disposed of in a suitable manner. Biological stabilization may be accomplished by either aerobic or anaerobic digestion. After digestion, sludge-drying beds are usually used.
In modern sewage treatment plants, mechanical dewatering of sludge by vacuum filters, centrifuges, or other devices is becoming widespread. The dewatered sludge, then, may be heat-dried, if it is to be reclaimed, or it may be incinerated. In large communities where large amounts of sludge are produced, mechanical dewatering and incineration are commonly practiced. But there are many smaller communities, rural areas, etc., which have economic constraints and which use the sand bed method to dewater sewage. There is a great need to make the sand bed method more economical by reducing the time for drying waste material (sludge) from the primary-treated sewage effluent and by reducing the time for drying the sludge. Reduced drying time would allow reduction of the size of the sand beds needed.
Early sludge treatment schemes included plain sedimentation, followed by chemical precipitation or sedimentation aided by flocculation chemicals. Chemical precipitation fell into disuse, but may be making a comeback. Nowadays, chemicals are often added to the sewage to promote the coagulation of the finer suspended solids, so that these solids become heavy enough to settle in sedimentation in the primary treatment stage. Typical chemical coagulants in the flocculation of sewage are alum, polymers, ferric sulfate, ferric chloride and lime.
Chlorine is often used to minimize odors from sedimentation tanks and in the final effluent as a disinfectant.
U.S. Pat. No. 5,248,416 (Howard) discloses a sewage treatment system which presents a main flow line and a recirculating line, the former for floc which has appreciated in size due to the addition of a polymer and to passage through an area of agitation/turbulence, and the latter for the return of small sized floc to the agitator/turbulence area for size increase. The passageways of the system include movable flaps which serve recirculation purposes, and a ledge or flutter for current creation and floc build-up. Raw liquid sewage enters the system, whereas the outlet leads to a belt press and/or a dry bed to cake the resulting sludge. More specifically, the apparatus for flocculating fluids containing suspended solids comprises conduit means for conducting the fluid to an outlet in the conduit means. There is means introducing a flock-producing agent into the fluid in the conduit means, a vertical drop in the conduit means downstream from the means introducing the flock-producing agent, and a movable mounted ledge means in the vertical drop which serves to increase turbulence and to increase the size of accumulating floc in the fluid. There is a vertical rise in said conduit means, downstream from the vertical drop leading to the outlet. The conduit means includes means connecting the vertical drop to the vertical rise, and there are circulation passageway means connecting the vertical rise to the vertical drop for recirculating smaller size flock to the vertical drop.
In Howard, it is said that a particular feature is that no mixer equipment is required. Polymers are injected into the raw sewage, causing water to separate from the raw sewage during the procedure, resulting in floc build-up. The latter is caused when the polymers begin dissolving with the result that a film of concentrated polymer solution builds up about the polymer particles, forming aggregates or agglomerations, identified as "flocks". Turbulence is a key factor, where such is said to be accomplished through a ledge (which flutters) located in the vertical drop conduit and a series of movable flaps disposed within the recirculating conduit. The singular stated purpose of the Howard scheme is to create flock, i.e., solids with a minimum of water content, through separation. Restated otherwise, the Howard scheme, through turbulence or tumbler-mixer action, is said to create additional floc (of a larger size) which goes to output, whereas smaller floc is caused to recirculate said increase, thereby, in size for repeated passage to output.